I think the idea is not to keep people from drinking as much soda as they want but stopping them from drinking more of it than they would want to if the sizes on offer weren't so huge. Europeans are usually impressed by the supersizes available in American supermarkets. It's an age old diet trick to use a smaller dish because psychologically your food will look like more, supermarkets have big shopping carts because that way your shopping looks like it is little and you could use some more. Big portions tempt people to eat or drink more than they usually would. Sure, nobody is forcing them but it tempts them and manipulates them into doing something stupid that is harmful to them, their families and those supporting their health care.
This is not about stopping you to drink all the soda you want it is about getting you to think for a moment before you mindlessly consume more than you normally would and more than you would have wanted to a few years ago before portions became so large.